I read the review from dpreview on both cams, have pros and cons, hard to decide too.
But when I did a comparison using the dpreview tool, there are more pros on D5000 than A550.
Tough lei.
This is a comparison between the 2 from a review:
Nikon’s D5000 is another best-selling mid-range DSLR, so represents another key rival for the A550. Both cameras feature articulated screens with Live View, 18-55mm kit lenses and HDMI ports, but like the Canon above, there are many differences to weigh-up.
Once again, the biggest feature in the D5000’s favour is a movie mode, which shoots at 720p in 24fps. In addition, its screen may be smaller and less detailed, but enjoys greater articulation, turning to the side or front for crafty self-portraits, not to mention back on itself for protection. The D5000’s viewfinder features on-demand grid lines, there’s a more sophisticated 11-point AF system, superior metering, interval-shooting, a time-lapse movie mode, and while it’s a personal thing, we also preferred its ergonomics.
In its favour, the Alpha A550 features built-in stabilisation which works with any lens you attach, faster continuous shooting at 5fps with AF or 7fps without, a choice of two Live View modes, one of which is much faster than the Nikon while additionally supporting continuous AF, double the maximum sensitivity, and a face detection system which is actually usable thanks to the phase-change AF behind it. The battery also delivers an accurate percentage of charge remaining, there’s an optional battery grip and also in-camera HDR capture. The A550 also features two extra Megapixels, although this didn’t make a great deal of difference in our real-life tests.
So once again the Sony stacks-up well, with video recording being the only really major feature lacking in comparison to the D5000. But again there’s a number of small but valuable benefits to the D5000 which may make it preferable overall, especially at the lower kit price.